1112.12 Discharge Piping — 2025 California Mechanical Code

The piping used for pressure-relief device discharge shall be in accordance with Section 1112.12.1 through Section 1112.12.5. [ASHRAE 15:9.7.9]

1112.12.1 Piping Connection. Piping connected to the discharge side of a fusible plug or rupture member shall have provisions to prevent plugging of the pipe upon operation of a fusible plug or rupture member.

[ASHRAE 15:9.7.9.1]

1112.12.2 Pipe Size. The size of the discharge pipe from the pressure-relief device or fusible plug shall be not less than the outlet size of the pressure-relief device or fusible plug. [ASHRAE 15:9.7.9.2]

1112.12.3 Maximum Length. The maximum length of the discharge piping installed on the outlet of pressure-relief devices and fusible plugs discharging to the atmosphere shall be determined in accordance with Section 1112.12.4 and Section 1112.12.5. {ASHRAE 15:9.7.9.3}

1112.12.4 Design Back Pressure. The design back pressure due to flow in the discharge piping at the outlet of pressure relief devices and fusible plugs, discharging to atmosphere, shall be limited by the allowable equivalent length of piping determined in accordance with Equation 1112.12.4(1).

[Equation 1112.12.4(1)]

L= 0.2146 - d [5] ( P [2] 0 P 22 ) d - ln ( P0P2 )

f - C [2] r 6 - f

Where:

L = Equivalent length of discharge piping, feet.

Cr = Rated capacity as stamped on the pressure relief device in pounds per minute (lb/min), or in standard cubic feet per minute (SCFM) multiplied by 0.0764, or as calculated in Section 1112.14 for a rupture member or fusible plug, or as adjusted for reduced capacity due to piping in accordance with the manufacturer of the device, or as adjusted for reduced capacity due to piping as estimated by an approved method.

f = Moody friction factor in fully turbulent flow.

d = Inside diameter of pipe or tube, inches.

ln = Natural logarithm.

P2 = Absolute pressure at outlet of discharge piping, psia.

P0 = Allowed back pressure (absolute) at the outlet of pressure relief device, psia.

22 ) d - ln (

L=

0.2146 - d [5] ( P [2] 0 P

( P [2] 0 P 22 ) d - ln ( P0P2 )

f - C [2] r 6 - f

Unless the maximum allowable back pressure ( P0 ) is specified by the relief valve manufacturer, the following maximum allowable back pressure values shall be used for P0, where P is the set pressure and Pa is atmospheric pressure at the nominal elevation of the installation (see Table 1112.12.4):

(1) For conventional relief valves: 15 percent of set

pressure:

P0 = (0.15• P )+ Pa [Equation 1112.12.4(2)] (2) For balanced relief valves: 25 percent of set pres sure:

P0 = (0.25• P )+ Pa [Equation 1112.12.4(3)]

(3) For rupture disks alone, fusible plugs, or pilot-operated relief devices: 50 percent of set pressure:

P0 = (0.50• P )+ Pa [Equation 1112.12.4(4)]

For fusible plugs, P shall be the saturated absolute pressure for the stamped temperature melting point of the fusible plug or the critical pressure of the refrigerant used, whichever is smaller. [ASHRAE 15:9.7.9.3.1, 9.7.9.3.2]

TABLE 1112.12.4

ATMOSPHERIC PRESSURE AT NOMINAL INSTALLATION ELEVATION ( Pa )

[ASHRAE 15: TABLE 9-2]

For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa

1112.12.5 Simultaneous Operation. When outlets of two or more relief devices or fusible plugs, which are expected to operate simultaneously, connect to a common discharge pipe, the common pipe shall be sized large enough to prevent the back pressure at each pressure-relief device from exceeding the maximum allowable back pressure in accordance with Section 1112.12.4.

[ASHRAE 15:9.7.9.3.3]